The present invention relates to improved multi-layer sheet materials and more particularly to a polymeric multi-layer material particularly suited for use in making containers for food products to be cooked or reheated in ovens.
All-plastic and plastic-paper laminants for use in making packages or containers are well known. Such laminants are disclosed in U.S. Pat. No. 4,183,435, assigned to the assignee of the present invention. The materials disclosed therein are for the most part intended to be used in making food trays and like containers where the nonporous nature of the material is the critical property. However, as U.S. Pat. No. 4,183,435 points out, a new market or use for such materials is developing. Microwave ovens are becoming more popular with consumers because of the speed at which such ovens can cook or reheat food and because of the lower energy requirements of such ovens relative to conventional ovens. Because microwave ovens cook or reheat food from the inside out, the food containers are not subjected to the same temperatures as they are in conventional ovens. Containers used in microwave ovens are heated only by heat conducted from the food being cooked. Since the temperature demands on the containers are lower, consideration has been given to the use of all-plastic or partially-plastic containers for pre-packed frozen foods in place of the relatively more expensive containers in which frozen food products are now typically packaged.
Thus, U.S. Pat. No. 4,183,435 discloses a novel multi-layer material particularly suitable for containers to be used in microwave ovens. The material includes a core layer of foamed, high density polymeric material and surface layers on both sides of the core layer. The relatively thin surface layers are formed from a polyester material such as polyethylene terephthalate. The core layer provides structural integrity for the material.
While the multi-layer sheet material and tray formed therefrom as disclosed in U.S. Pat. No. 4,183,435 have proven to be entirely satisfactory for the purpose intended, one difficulty has been noted. If any printing is desired on the completed tray, it is usually placed on the back side of the surface layer and when the surface layer is attached to the core layer, the printing is visible through the transparent surface layer. However, when the surface layer is in a roll prior to being applied to the core layer, it has been found that the ink transfers from the back side of the surface layer to the interior side by surface-to-surface contact while in the roll form. When unrolled, the ink that has been transferred to the interior surface of the roll becomes exposed on the outer surface of the outer layer thus coming in direct contact with the food that is placed therein. Since it is exceedingly difficult, if not impossible, to obtain inks which are permitted for direct contact with food, it is difficult to construct such trays with printing thereon that will not come in contact with the food.